The drag experienced by a vehicle at speed is the largest contributor to that vehicle's fuel consumption. If the drag force on a vehicle can be reduced, the amount of fuel required to run the vehicle will be less, thereby reducing both the running costs for the vehicle and the environmental damage caused by the burning of said fuel.
When a vehicle is moving, the air it travels through causes the vehicle to experience a drag force. This aerodynamic drag has two main components: friction drag and pressure drag. The first component, friction drag, arises because, as the vehicle moves through the air, the air nearest the vehicle adheres to the surface of the vehicle and travels with it. Friction then occurs between this air and the layers of air through which the vehicle is moving, producing the frictional component of the drag force. This air around the vehicle which contributes to the frictional drag is called the boundary layer. The thickness of the boundary layer, and whether it is laminar or turbulent, varies along the length of the vehicle and is dependent on parameters such as the viscosity of the air and the velocity and geometry of the vehicle.
Pressure drag is caused by the difference in pressure on the forward facing surface of a vehicle (i.e. the surface which is moving into the air when the vehicle is in motion) and on the backward facing surface of the vehicle. The forward facing surface will hereafter be referred to as the front of the vehicle, and the backward facing surface will hereafter be referred to as the back or rear of the vehicle. The lower the pressure at the rear of the vehicle as compared to the front, the greater the pressure drag force which opposes the motion of the vehicle. The pressure at the rear of the vehicle is affected by the type of air flow behind the vehicle; for example, large turbulent wakes may lead to a higher pressure differential than smaller wakes. Whilst reducing the strength of the wake behind the vehicle through streamlining of the vehicle decreases the contribution of pressure drag to the overall drag force, streamlining also increases the contribution from friction drag, as the wetted area of the vehicle increases.
A bluff body is a body for which the aerodynamic drag is dominated by pressure drag (for example, a lorry, truck, large goods vehicle (LGV), van or 4×4), whereas the drag on a streamlined body (for example, an aeroplane) is dominated by friction drag. A bluff body typically has a high contribution from pressure drag because the boundary layer cannot easily follow the contours of the body and thus separates from the body rather than continuing to adhere to its surface. As the air flow separates from the bluff body, eddies and vortices form and create a wake. For an articulated lorry, pressure drag can be as much as 90% of the total aerodynamic drag. Therefore, reducing the pressure drag is important for reducing the fuel costs for bluff body vehicles such as LGVs and trucks.
As discussed above, pressure drag is proportional to the pressure difference between the front and rear of a bluff body. For a moving vehicle, the pressure difference is equal to the pressure on the front of the vehicle due to the air it is moving into minus the pressure on the opposite, rear, end of the vehicle. Therefore, there are three ways of reducing the pressure drag of a vehicle, or other bluff body: reducing the pressure at the front of the vehicle, increasing the pressure at the rear of the vehicle, or a combination of the two.
WO 2014/016618 provides an example of an active apparatus for reducing pressure drag comprising oscillating means arranged to draw in and eject fluid from a cavity. The ejected fluid entrains fluid from the environment in order to increase the pressure at the rear of a vehicle comprising the apparatus, and thus reduce the pressure drag.